1. Field of the Invention
The present invention relates to a diagnostic apparatus for an evaporative emission control system and, more specifically, to a leak diagnosis of an evaporative emission control system including a fuel tank.
2. Description of the Related Art
Known is an internal combustion engine including an evaporative emission control system for preventing evaporated fuel in a fuel tank to be discharged in the atmosphere. In this system, evaporated fuel (evaporation gas) generated in the fuel tank is temporarily adsorbed by an adsorbent filled in a canister, and the adsorbed evaporated fuel is discharged to an intake system in an internal combustion engine via a purge passage (or line) under predetermined operating conditions. However, when a part of the system is broken or disrupted for some reasons, the evaporated fuel is discharged to the atmosphere. In order to prevent such a matter (or inconveniency), performed is leak diagnosis for determining presence of a leak in the evaporative emission control system. (for example, see JP-A-2001-41116)
In this leak diagnosis, a purge control valve (purge control solenoid valve) is fully closed first, and then a canister closing valve (drain valve) is fully closed, whereby a closed purge passage sealed from the atmospheric pressure is formed, and measured is a first pressure change amount P1 due to a generation of the fuel evaporation gas under a positive atmospheric pressure. Then, the purge control valve is opened, and hence a negative pressure in the intake pipe is introduced into the fuel tank. When the purge control valve is closed again, formed is the closed purge passage sealed by a predetermined negative pressure, whereby measured is a second pressure change amount P2 due to the generation of the fuel evaporation gas under the negative pressure. Then, a pressure leak presence is determined on the basis of the relation between the measured pressure change amounts P1 and P2 from the evaporative emission control system.
However, in the technology disclosed in the aforementioned JP-A-2001-41116, since the pressure change amounts P1, P2 are measured by sealing the evaporative emission control system in different pressure states respectively, it may be difficult to perform accurate leak diagnosis.
In other words, when the evaporative emission control system is sealed in different pressure states, a generated evaporation gas amount varies significantly even when such conditions as the amount of remaining fuel or the temperature are the same. Therefore, when the pressure change amounts P1, P2 are independently measured under such unstable conditions, it is difficult to determine the accurate leak.